Multivibrator count down circuits



July 18, 1950 v c. H. SMITH, JR., EI'AL 2,515,271

MULTIVIBRATOR coum' DOWN CIRCUITS Filed April 28, 1945 2 Sheets-Sheet 1 Ila-LL QwuwWo b CARL H. SMITH JR. CONRAD H. HQEPPNER y 1950 c. H. SMITH, JR.. EI'AL 2,515,271

MULTIVIBRATOR COUNT DOWN CIRCUITS Filed April 28, 1945 2 Sheets-Sheet 2 IIEE SrWtZ YM CARL H.-SM|TH, JR. CONRAD H. HOEPPNER Patented July 18, 1950 MULTIVIBRATOR DOWN CIRCUITS Carl H. Smith, Jr., Arlington, Va., and Conrad H. Hoeppner, Washington, D. C.

Application April 28, 1945, Serial No. 590,861

(Granted under the act of March 3, 1883, as amended April 30, 1928; 370 O. G. 757) 4 Claims.

This invention relates in general to an electronic frequency divider and inparticular to one wherein the ratio of the input frequency to the output frequency is maintained constant over a wide range variation in'the former.

It is an object of this invention to provide a frequency divider circuit wherein each output signal from the circuit appears in time coincidence wits the input signal which initiates the frequency division cycle.

It is another object of this invention to provide a frequency divider circuit of the foregoing type with additional means provided for maintaining a constant frequency division factor over a wide range of input signal frequencies.

Another object of this invention is to provide a frequency divider circuit whose output frequency will change in accordance with the input frequency and in such a manner that the output frequency will not exceed a certain maximum value.

Another object of this invention is to provide a signal responsive device for producing a symmetrical square-wave voltage at the frequency of an applied signal.

Another object of this invention is to provide a means in the foregoing device for maintaining a symmetrical output wave over a wide range of variation in the frequency of the applied signal.

Other objects and features of the present invention will become apparent upon a careful consideration of the following detailed description when taken with the accompanying drawings.

Fig. 1 is a schematic diagram showing the preferred embodiment of the invention;

Fig. 2 is ,a series of waveforms illustrative of the action of the circuit of Fig. 1 when operating as. a frequency divider;

Fig. 3 is a. schematic diagram of a iflcation of the invention.

For purposes of illustration a frequency-dividing circuit of atype especially adapted for pulse operation is'shown in Fig. 1. This circuit comprises a first clipper-amplifier [0, a multivibrator including tubes H and I2, and a second clipperamplifier 13. It is to be understood that this arrangement is representative of a'preferred embodiment of the invention and'may be modified in various respects without'departing from the spirit of the invention. For example, in Fig. l the multivibrator is represented as being of the cathodecoupled variety when actually it is only necessary that it be of the type which has one stable electrical state but will support for a prety pical mod-- determined perlod of time-and in'respon'setoian input signal a second electrical state. Furthermore, if desired, the arrangement may be driven from a sine wave source in lieu of the pulse source drive as hereinafter described and shown;

In the case where the frequency divider is driven from a pulse source, a series of positive pulses obtained from the source, not shown, is applied to the grid of the clipper-amplifier It. This tube serves primarily as a means for shaping and polarizing the input signal applied to the multivibrator. To this end tube I0 is normally operated below cut-01f potential by means of the positive bias voltage applied to its cathode through the voltage-dividing resistances M and I5. To couple the output from tube I0 to the multivibrator, a resistance I7 is inserted in the plate circuit of both tube Ill and tube ll. Then the positive pulses obtained from the pulse source and applied to tube l0, if of suflicient amplitude to raise the grid of the latter above its cut-ofl potential, will cause a pulsating plate current to pass through the tube. This pulsating current produces at the plate of tube ll, due to the resistance IT, a series of negative voltage pulses which are applied through capacitance 20 to the grid of tube 12 in the multivibrator circuit, instituting a change in the electrical state of the multivibrator. As will be seen hereinafter, this change in the electrical state of the multivibrator represents the start of the frequency division cycle. To prevent the operation of tube l0 from affecting the performance of the multivibrator, an isolating resistance l6 of, for example, the same magnitude as resistance [1 is inserted between the plates of tubes In and I I.

In the present case the initial or stable state of the multivibrator exists when'tube I2 is conducting and tube II is non-conducting. To this endthe plate resistance I 8 of tube l2- and the common cathode resistance [9 of tube H and i2 are chosen so that in the quiescent state conduction by tube 12 produces a positive potential 'at the cathodes of tubes H and I2 which is of sufficient magnitude to prevent conductionrbytube H. With this arrangement existing in the multivibrator a negative pulse applied to the grid of tube 12 through capacitance 20, if of sufiicient amplitudeto stop the flow of current through tube l2, will lower the potential at the cathode of tube II and thereby cause that tube to conduct. The resulting drop in potential at the plate of tube H is applied through capacitance 2B'to drive the grid of tube l2 further beyond cut-off potential; Capacitance 20 then begins as exponential discharge throughtube II and resistance input signal.

2| permitting a gradual rise in the potential on the grid of tube 12. Eventually the grid of tube l2 rises to cut-01f potential and tube l2 returns to conduction. The period for which tube !2 is held non-conducting is dependent upon two factors, namely, the time constant of the discharge path of capacitance 2B and-the magnitude of the drop in potential at the plate of tube l I as conduction by that tube is initiated.

Frequency division is thus accomplished in the multivibrator withthenon-conducting period of tube l2 governing the frequency division factor. To regulate the frequency division factor of the system the grid coupling resistance 2i of tube i2 is made variable and therefore the cut-off period of this tube is made variable. To illustrate more precisely the operation of the circuit, reference is had to the waveforms shown in Fig, 2. Waveform A is representative of the input signal applied to the grid of tube l6. Waveform B is representative of the grid voltage variation of tube 12. As shown, the initial pulse in waveform A'triggers the multivibrator to cut-off tube l2. Thereafterseven additional pulses occur during theperiod that tube I2 is non-conducting. At some instant between the eighth and ninth pulses the tube l2 returns to conduction only to be again cut-off by the ninth pulse. A frequency'division is thus obtained.

As aforementioned, one of the objects of the invention is to provide a frequency divider system wherein the output signal occurs in time coincidence with the start of the frequency division cycle. For this purpose the output of tube i2 is applied through a differentiating circuit comprising capacitance 22 and resistance 23 to the grid of a second clipper-amplifier 13. As tube i2 passes from conduction to non-conduction and back again, the voltage at its plateswill'vary due the endof each division cycle. When such a voitage. wave is subjected'to differentiation, as by the circuit comprisin capacitance 22 and resistance 23., a voltage variation, such as that shownby waveform D, will appear across the resistance. This voltage'is applied to the grid of tube 13. Tube 43 is normally operated below cut-off potential by'means of a positive voltage maintained at its cathode by the voltage-dividing resistances fi l and 25. Therefore only the positive peaks of waveform D, which correspond to t-he return to non-conduction" of tube 1 2, will cause tube 3; to conduct. Waveform Ein Fig. 2 is' illustrativeof the output signal from the circuit as taken at'the plate of tube 13 across resistance 25:; As may be observed from waveform-s B, C, D, and E, this'output signal .isintime coincidence with the start of each frequency division cycle and is therefore'i-n time coincidence with a-pulse of'the 1 Means are further providedinthe multivibrator for maintaining a' relatively constantfreque'nc'y division factor despite changes in the input signal frequency. Broadly thismeans'funw tions in response 'to changes in' the'input'signa'l frequency, controllin the potential below -c1it 01f to'which the grid of the normally conducting tube, in the present illustration tube! 2, is driven and thereby the period during which thistube is held non-conducting. In operation, any change in the 'frequency':of"the input signal altersyas factor of -8 4 hereinafter described, and in an inverse sense the potential below cut-01f to which the grid of tube i2 is driven.

In particular this compensating action is achieved through a filtering circuit which in the present'case comprises capacitance 2B in combination with resistances ll, 2?. and the plate resistance of tube H. The time constants of the charge and discharge paths of this capacitance,

determined respectively by resistance El and resistance il in combination with the plate resistance of tube i i, are made long with respect to the longest desired period between output pulses, which in effect is long compared to the time constant of the combination of resistance 2i and capacitance 2B. Thus the potential at the juncture of resistances i7 and 21 is unable to change appreciably during any individual frequency division cycle.

To illustrate the corrective action, consider a case in which the frequency of th input signal has suddenly increased. Th'm wiil not causeian immediate change in output signal frequency, but will result in a higher frequency division factor. Since, as previously shown by'waveform B :of Fig. 2, the maximum conduction period of tube i2 is that period between two consecutive input signals, this tube willconduct fora shorter period of time during each frequency division cycle.- This results in a higher duty cycle (ratio of time conducting to time non-conducting) for tube l l. The higher average current drawn by tube 'M pro duces a greater voltage drop acrossresistancezl lowering the potential at the juncture of resistances l l and El and therefore the effective platesupply voltage for the tube. This reduction in plate-supply voltage for tube '23! reduces the amplitude of the drop in potential produced at the plate of l l as that tube goes from cut-off to corn duction, consequently the grid of tube lZ is not driven so far negative and will therefore rise to cut-off potential in a short'er'period of time: This reduces the number of input signals which can occur during any non-conducting period of tube l2,

thereby restoring the frequency division factorjto its former value.

This corrective action, h 'owever, cannot cont'inue indefinitely. Eventually the plate-voltage experienced.

.A .similaraction takes place in the opposite direction upon decreases in the Tinputsignal frequency. In this case the voltage atI-thejuncture point of resistances l1 and .21 increasesto cause the grid of tube l2 to be driven further negative astube H is rendered conducting. Here too,;a limiting action is experienced, since 'it;is;imposjsible for the potential at the'junctionof resist 'ances I1, 21 to rise above the plate-supply voltage. 1 I

' .By making-slight changes'iin the timeconstant of some 'of the resistance-capacitance circuits in the circuit Of Fig. 1, the latter may'serve-as a means for'producing a symmetrical square-wave output voltage in synchronis'm'with and at the frequency of an input signal. Furthermore, by the action of the filtering circuit this square-wave voltage output may be maintained symmetrical over a wide range of variations" in the inputsignal frequency.

To achieve this result, the time constant of the discharge path of capacitance is'adjusted by means of the variable resistance zlso'that'the cut-off period of tube I2 will be equal to the half period of the mean input signal" frequency with the potential at the junction'of' I1 and 21' at approximately the mid-poi'nt in the corrective voltage range. Thus a 50 percent duty cycle will be gmaintained for both tubes II and I2 producing a symmetrical square wave at the 'plate'of tube I2. In using the circuit as a square-wave generator, the circuit coupling the plate .of tube I2 to the grid of tube I3, comprising capacitance 22 and jresistance 23, is made of the long timeconstant variety. As previously described, tube I3 is maintained in a normally cut-off condition by means of voltage-divider action between resistances 24 and 25. It becomes conducting during the period in which tube I2 is cut off and serves merely to clean up the square wave produced at the plate of tube I2.

In the production of the square wave by the multivibrator II-IZ, tube I2, which in the quiescent state would be conducting as previously described, is cut off by a negative pulse from the plate of tube ll produced as a result ofthe conduction current of tube I0 through the resistance I'I. After a predetermined period equal in time to the half period of the desiredsquare-wave output, tube I2 is returned to conduction by the discharge of capacitance 20. It then remains in this condition until cut oif by a second negative pulse from the plate oftube I I.

To compensate for changes in the frequency of the input keying signal, the size-of theelements in the filtering circuit are again chosen so that the time constants of both the charge and discharge paths of capacitance 28 are several times as long as the half period of the desired output square wave, or in eifect the time constant formed by capacitance 2E) and resistance 2 I.

To illustrate the compensating action of the filter circuit, consider the condition in which the frequency of the input signal has increased. This will not immediately affect the cut-off period of tube I2, since this is determined primarily by the discharge period of capacitance 20, but it will reduce the cut-off period of tube I I, thus increasing the average duty cycle of tube I I and the average current drawn by that tube through resistance 21. This increased average current will lower the average potential existing at the junction of resistances I! and 2'! efiectively lowering the plate-supply voltage for tube II. Thus a smaller voltage drop will be produced at the plate of tube II as that tube goes from cut-off to conducting and thereby reducing the negative potential from which the grid of tube I2 must recover before that tube can begin conducting. The action is therefore to reduce the cut-off period of tube I2 so that an essentially square- Wave output is again realized. A similar action in the opposite direction also ensues upon a decrease in the frequency of the input signal. For optimum performance of the circuit it is desirable to adjust the time constant associated prior art. I

electrodes I tube connected to certain' electrodes of the I other tube so as to form a multivibratorfsaid with' "the discharge path "of "capacitance 20 so that tube input frequency range,'be cut off for. the -;half

I2 will, at the -mid-point in the period of the input trigger when thepotential at the junction of I! and 21' is'at the' inid-point in its corrective range. I

An obvious modification of the circuit'shown in Fig. l is disclosed in Fig. 3. The sole difference in the two circuits resides in the type of 'multivibrator 'used to accomplish the frequency division. In the caseof'the circuit shown in Fig.

produced .at the plate or tube II as that tube passes from non-conduction to conduction.

. Fromthe'foregoing description it isapparent that many modifications of the invention are'po'ssible without exceeding the spirit of 'thelinvention. Therefore this invention is not to be limited except as is necessitated by the spirit of the The invention described herein may be manufactured and used by or for theGovernment of the United States of America for governmental purposes without the payment ofean y royalties I thereon 'or therefor.

What is claimed is: v I p 1. The combination, comprising. a pair'of. tubes having at least a plate, cathode','and control. grid with certain electrodes; of .one

plate electrode of said other tube being the ,input terminal of said multivibrator, said mu 1 'tivibrator' having one stable -electrical state wherein said one tube is normally held conducting and said other tube non-conducting but will support for a finite period of time a second electrical state wherein said other tube is held conducting and said one tube non-conducting, said second electrical state being established in response to an input signal applied to said multivibrator to cause said other tube to conduct whereby the grid of said one tube, by the multivibrator action, is driven below cut-off and held there for a predetermined period of time, said predetermined period of time being in part a function of the voltage below cut-off that the grid of said one tube is driven, separate plate supply paths for said tubes, and a filter circuit connected in the plate supply path of said other tube and responsive to increases or decreases in the input signal frequency to inversely alter the voltage below cut-off to which the grid of said one tube is driven, said filter circuit having a large time constant relative to the period over which said second electrical state may exist.

2. The combination, comprising a pair of tubes having at least a plate, cathode, and control grid electrodes with certain electrodes of one tube connected to certain electrodes of the other tube so as to form a multivibrator, said multivibrator having one stable electrical state wherein said one tube is normally held conduct- Similarly the multivibrator'supports.

support for a finite period of time a second elec- .:trica1 :state wherein said other tube is held conducting and said one tube non-conducting, said second .electrical state being established in response to an input signal applied to said multivibrator tocause said other tube to conduct whereby'the grid of said one tube, by the multivibrator action, is driven below cut-oft and held there ,for a predetermined period of time, said predetermined period of time being in part a func- =tion; of the voltage below cut-off that the grid of said-one tube is driven, anda plate voltage supp yrpath for said other tube, said path in- ;cluding a resistance-capacitance filter circuit operative to vary the voltage at the plate of this tube in amanner proportional to the ratio of the non-conducting time to the conducting time of this tube thereby-to alter in a like manner the "voltage below cut-off to which thegrid of said onetube is driven, said filter circuit having a ,large ,time constant relative to the period over which said second electricalstate may exist. -3. .The combination, comprising apair of tubes having at least .a plate, cathode, and control grid electrodes with certain electrodes of one tube connected to certain electrodes of the other "tube so to form a multivibrator, said multivibrator havingone stable electricalstatewherein said one tube is normally held conducting and vsaidother tube non-conducting but will support for a finite period of time a second electrical state wherein said other tube held conducting and 'said one tubenon-conducting, said second electrical state being established in response to an inputsignal applied to said multivibrator to cause said other tube to conduct whereby the grid of said one tube, by the multivibrator action, is

.driven below cut-ofi and held there for a predecircuit connected in the plate circuit of said other tube operative to vary the voltage at the plate of said other tube in a manner proportional to the 8 ratio of the non-conducting period to'the conducting-period of this tube thereby to alter in a like manner the voltage below -cut-ofi to which the grid of said-one tube is driven, said filter circuit having a large time constant relative to theperiod over which said second electrical state may exist. I I v 4. The combination comprising, 'a two tube trigger circuit of the type normally in a first state of conductivity but operative in response to an external signal to assume a second state of conductivity, each of saidtubes having at least plate,grid, and cathode electrodes, a time constant-circuit connecting the plate of one tube to the grid of the other to control the duration of said second state asa function of its time constant and thevoltage applied to the plate of said one tube, a voltage-supply source, and a filter circuit connected between said supply source and the plate of said one tube to vary the voltage applied to the plate of said one tube in accordance with changes in the relative time ratio of the two conductivity states, said filter circuit having a large time constant relative to the duration of the second state of conductivity. I

- CARL H. SMITH, JR.

CONRAD H. HOEPPNER.

REFERENCES CITED The following references are of record inthe file of this patent:

UNITED STATES PATENTS Number Name Date 1,672,215 Heising June 5, 1928 2,113,011 White Apr. 5, 1938 2,185,363 White Jan; 2, 1940 2,226,706 Cawein Dec. 31', 1940 2,377,894 McCool June 12, 1945 2,395,368 Bull Feb. 19,1946

2,401,647 Kahn June 4, 1946 FOREIGN PATENTS Number Country Date 485,934 Great Britain May 26, 1938 

